Qualitative Analysis



<< HOME

 

General

 

Qualitative analysis is a technique to determine the identity of cations, anions and/ or gases present by conducting specific tests and recording the observations obtained.

 

[QA Table] - will be provided in the exam paper if needed

 

 

Example of an O-level QA question:

 

Conduct of QA Tests

  • Scan through the entire practical paper.


    For each part, write down: (1) what can the test be used to identify? (2) what are the observations that you should look out for?


    e.g. Add sodium hydroxide solution, dropwise with shaking, until you observe no further change.


    (1) Test for cations. (2) Look out for (I) colour of ppt and (II) whether ppt dissolves in excess sodium hydroxide.

  • If a gas is expected to be formed, try to anticipate prepare the test for gas in advanced.


    e.g. To some solid S in a test tube, add dilute hydrochloric acid to a depth of 2 cm. Test for any gas evolved.


    Since an acid was added, the gas produced must be either hydrogen or carbon dioxide (recall the reactions of acids).


    If the solid is grey/ silvery, it is likely to be a metal → get ready lighted wooden splint to test for H2 upon addition of acid.


    If the solid is white or other colours, it is likely to be a carbonate → get ready limewater-delivery tube set-up to test for CO2 upon addition of acid.

  • When adding aq. NaOH or aq. NH3 for the testing of cations, it must be done dropwise and look out for any ppt formed.


    You may miss any ppt formed if aq. NaOH or aq. NH3 is added too fast.

  • What is the difference between gentle and strong heating?


    Gentle heating is carried out with the bunsen burner airhole half-open (luminous flame).


    It is usually done on solutions and you need to hover the test tube over the flame to avoid the solution boiling over. Once you feel vibration in the test tube, briefly move it away from the flame otherwise the solution will spurt out.


    Strong heating is carried out with the bunsen burner airhole fully-open (non-luminous flame).


    It is usually done on solids and you need to place the test tube over the flame for an extended period (no need to hover). If you don't heat the solid strong enough, you may miss out on some expected observations.

Recording of Observations

 

There are many observations! How do I know which one to record?


For each test, you should write down (wherever possible):
(1) what can the test be used to identify?
(2) what are the observations that you should look out for?

e.g. add nitric acid followed by barium nitrate.
(1) test for sulfate ions.
(2) look out for white ppt formed.

For a particular test, you should only write down the observations that you are supposed to look out for.


observations: white ppt formed [deduction: sulfate ion present] OR
observations: no ppt formed (no visible change) [deduction: sulfate ion not present]

 

[Link] Sample list of observations

 

 

Test for Gases
  • Whenever a gas is produced, write the following 4 points:
    (1) effervescence
    (2) colour and smell of gas (e.g. colourless, odourless gas produced)
    (3) gas test and observations (e.g. gas extinguished lighted splint with pop sound)
    (4) identity of gas (e.g. gas is hydrogen gas)
  • Include ‘moist or damp’ when describing litmus paper colour change (e.g. moist red litmus paper turned blue)
  • When describing colour change, state both the initial and final colour (e.g. potassium manganate (VII) solution turned from purple to colourless)

 

Test for Cations
  • What observations do we need to write down in our answers?
    Write obsevations specific to what is the procedure testing for.
    e.g. addition of NaOH is to test for cations; so we need to write down (1) whether ppt formed? (2) colour of ppt? (3) whether ppt dissolved in excess.
  • When a ppt dissolves in excess reagent, need to state the colour of the resulting solution. e.g. white ppt dissolved in excess NaOH to form a colourless solution.
  • When no ppt formed, write either ‘no visible change’ or ‘no ppt formed’.

 

Test for Oxidising/ Reducing agents (OA/RA)
  • Since OA and RA will give a colour change on undergoing reaction, we need to record the colour change for both the reagent and the solution separately
    e.g. add H+/KMnO4 to a solution containing Fe2+; KMnO4 turned from purple to colourless; solution turned from pale green (colourless) to yellow.

     

No comments:

Post a Comment